In a cell having a structure which houses in an outer case a cell element formed by stacking a positive electrode plate(s), a negative electrode plate(s) and a separator(s) together with an electrolyte solution, it is known that the impregnation condition of electrolyte solution is measured after the electrolyte solution injection (before charging) (e.g. Patent Publication 1). Specifically, a cell after the electrolyte solution injection (before charging) is irradiated with ultrasonic waves, and the electrolyte solution impregnation completion (impregnation condition) is determined based on the transmittance of the ultrasonic waves passing through the cell.
The impregnation condition inspection described in Patent Publication 1 is carried out before charging. However, there also exists a gas that would be newly generated by charging. Some of the bubbles from the gas may remain between electrodes. Since a cell having such faulty impregnation condition continues to flow through a production line until it is detected as a cell property deterioration (too small capacity, etc.) in a cell property inspection process at the final stage, it would become a factor of the production efficiency decline and the manufacturing cost increase.